1. Field of the Invention
The present invention relates to analytical probe stations, such as used in semiconductor manufacturing and testing procedures, laser cutters used standing alone or with the probe stations, and to multi-wavelength laser systems suitable for use in this environment.
2. Description of Related Art
Analytical probe stations are in widespread use in semiconductor manufacturing and design facilities. When a design engineer or failure analyst has to debug a circuit, it is most often done with the aid of an analytic-probe station. The probe stations typically include a base machine with a platen for mounting probes, probes for positioning on the platen, a chuck or chucks on which to mount a semiconductor or other subject of the probe station, a microscope bridge to support a microscope, and a microscope mounted on the microscope bridge. The probes contain microscopic probe needles which are used to check signals and make measurements at various locations in the integrated circuit.
The microscope has a field of view on the subject of the probe, so that a scientist or engineer can probe a semiconductor device or other component under direct visualization. A representative system is known as the SUSS PM 5 laboratory prober manufactured by Karl Suss, Waterbury Center, Vt. The laboratory prober is typically available with a variety of microscopes, including relatively low magnification stereoscopic systems to extremely high magnification probing microscopes.
Such probe stations are often used for analysis of integrated circuits, or other devices like liquid crystal display (LCD) panels, which comprise a plurality of layers of material. For instance, an integrated circuit may be formed on a semiconductor, with one or more polysilicon layers, one or more oxide or isolation layers, and one or more metal layers.
To be able to probe an integrated circuit, the protective passivation layer must be removed. This may be done using an ultrasonic cutter, a sturdy probe tip (by scratching), a plasma or chemical etching process, a focused ion beam system, or a laser system. The laser is pulsed through the microscope and can remove the passivation material to enable the engineer to probe the circuit. The laser may also be used to cut circuit lines to isolate or modify a circuit.
Similarly, in the manufacturing process for large-scale LCDs, shorts may appear at various locations. Since large LCDs are expensive, it is economical to repair these shorts. A laser is used to remove the short by focusing enough energy density on the short material to vaporize it. LCDs typically use indium tin oxide (ITO) for the nearly invisible conducting lines on the LCD screen. Chrome is also used on the borders as conductive buses. A color filter is used in color LCDs. The color filter material may also have manufacturing defects which can be repaired. ITO, chrome, and various color filter shorts may be repaired with a laser system.
Accordingly, some probe stations in the prior art have been coupled to lasers. One typical system in the prior art is known as the Xenon Laser Cutter, Model No. SUSS XLC. This system utilizes a pulsed xenon laser source which is directed to the device under test through special optics coupled with a high magnification microscope. The predominant wavelength of this system is in the green optical region, so it passes readily through the microscope optics. This single wavelength system is quite complex. The xenon laser must be mounted with the microscope, such that the output of the xenon laser is directed through the microscope optics. In a probe station environment, which already involves a large amount of instrumentation, the addition of a laser system makes the station much larger and more complicated. In addition, the expense of such laser systems has been quite high.
One limitation of a laser cutter system for a probe station which generates a single wavelength, is that the single wavelength may not be appropriate for cutting off certain types of layers of material. For instance, semiconductors typically have aluminum lines deposited on silicon wafers. There may be one, two, three, or four layers of metal lines separated by interlayer dielectrics. The whole device is then coated with a non-conductive passivation material to protect the circuit. The metal lines are typically aluminum, but may also be gold or titanium-tungsten. The passivation materials are typically silicon dioxide, silicon nitride, and polyimide.
In the semiconductor failure analysis market, the most universal wavelength is in the green region. Most metals absorb green laser energy very well and are usually very easily cut with one pulse. The green wavelength may be generated with a xenon laser, or with a frequency doubled Nd:YAG system. Most passivation materials are transparent to visible light as well as green laser energy. To remove passivation material which does not absorb green energy, one must heat up the underlying metal to a temperature which will "blow out" the passivation material. This can usually be accomplished if the underlying metal is of sufficient mass which will not vaporize when the laser pulse hits. It becomes very difficult to remove certain passivation materials such as nitride and polyimide when the metal line is small, or if one is trying to access a metal layer underlying a top metal layer, or the underlying material is silicon or polysilicon.
Certain passivation materials, specifically nitride and polyimide, can be removed directly with ultraviolet energy. These materials absorb UV energy directly and are slowly ablated using multiple low energy UV laser pulses. Unfortunately, silicon dioxide is not absorbing of most UV wavelengths (except for around 200 nanometers) and must be removed indirectly using the heating method described above. Infrared laser energy is widely used in the flat panel display repair market. Most of the materials used in this market absorb infrared wavelengths. However, some materials, such as chromium, and some color filter materials are more absorbing of the green wavelength. In this market, all of the material in the target area must be removed, and the cuts are usually relatively large, i.e., 5-40 microns.
The infrared wavelength also has application in the semiconductor analysis field. Silicon is generally transparent to infrared energy. This allows one to remove metal lines with less damage to the underlying silicon with infrared than can be accomplished with green energy. Using green energy, the cut line can short to the substrate because of heating of the silicon. This happens less often with infrared, making it an excellent complementary wavelength to green for semiconductor failure analysis.
The following table summarizes performance for some wavelengths on common materials.
______________________________________ 1064 nm (infrared) 532 nm (green) 355 nm (ultraviolet) ______________________________________ ITO ITO Nitride Chromium Chromium Polyimide Color filter Color filter Silicon Dioxide Nitride Polyimide (big cuts) SOG Poly-silicon Gold Aluminum Tungsten ______________________________________
However, the prior art has been unable to provide two wavelengths of light from a single laser system. Thus, the probe station or laser cutter which is capable of supplying more than one wavelength of light has used two or more separate laser systems, becoming very large and unwieldy. For instance, one prior art system has combined an excimer laser which supplies ultraviolet with a doubled YAG laser on a single probe station, such as a combination of the model LCM-308 EXCIMER LASER CUTTER ATTACHMENT with the laser of the model LCP GREEN YAG LASER CUTTER, both manufactured by Florod Corp. of Gardena, Calif. However, the excimer laser is a large bulky device, requiring complicated tubular wave guides to deliver the laser energy to the microscope optics. This system is very expensive and uses up valuable laboratory space.
Accordingly, there is a need for a multi-wavelength laser system for use with a probe station or laser cutter, which is economical, small in size, and efficient.